The measles outbreak continues to grow with an additional 19 confirmed cases in the last week, bringing the total number of measles cases to 121 since the U.S. Centers for Disease Control and Prevention began tracking the outbreak on Jan. 1.

The outbreak is believed to have begun with a group of unvaccinated people visiting Disneyland in California in December, but it has now spread to 17 states — the latest being Delaware, Michigan and Nevada — according to the CDC. Last week, five babies at an Illinois daycare center were diagnosed with measles, prompting national daycare center chain KinderCare to require all staff members working with newborns to be vaccinated.

Of the first 34 people with measles for whom the California Department of Public Health had vaccination records, only five had received both doses of the measles vaccine, according to the agency. One received just the first dose. Nationally, officials are seeing the same trend, said Dr. Anne Schuchat, who directs the National Center for Immunization and Respiratory Diseases, said during a news conference Thursday. Some of those 34 cases tracked by California may not be included in the 121 tally by the CDC because they were reported before Jan. 1.

“This is not a problem with the measles vaccine not working,” she said. “This is a problem of the measles vaccine not being used.”

The CDC is seeing more adult cases of measles than usual during this outbreak, Schuchat said, adding that children are getting the virus, too.

Cases have now been reported in Arizona, California, Colorado, Illinois, Minnesota, Michigan, Nebraska, New York, Oregon, Pennsylvania, South Dakota, Texas, Utah and Washington, according to the CDC. The agency issued a health advisory on Jan. 23, at which point the virus had only spread to six states beyond California and Mexico.

The United States last year reported its highest number of measles cases in two decades, with 644 cases as part of 20 separate outbreaks. Health officials attribute the spike to a measles outbreak in the Philippines and overseas travelers.

The 121 cases in the U.S. reported since Jan. 1 of this year are considered part of one outbreak.

The measles virus is contagious long before symptoms appear, and it is airborne, according to the CDC. One infected person with the measles can spread it to an average of 18 other people, and it can linger in the air and live on surfaces to spread after an infected person has left a room.